If it is an older house, it may not be too difficult to run Ethernet cable either below the floor or above the ceiling to a more central location. Then you could either 1) add a more capable wifi router (and turn off the Xfinity wifi), or 2) add another access point at the new location to provide a better wifi coverage.

Very old homes, sometimes have lots of plaster in the walls and that is problematic for wifi penetration.

No technical barrier here. For $10 you can see if it meets your needs